226 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



genera of hyopsodontids in the present fauna, Haplaletes, Litomylus, 

 and Litalestes. The number of known genera has nearly doubled 

 since Matthew completed his work, and the whole unwdeldy group 

 requires reconsideration. The genera now known and considered 

 hyopsodontid are as follows: 



Choeroclaenus (defined below): Type C. turgiduncnlus. Lower Paleocene. 



Oxyacodon: Tj^pe 0. apiculatus. Lower Paleocene. 



Mioclaenus: Type M. turgidus. Middle Paleocene. 



Elli-psodon: Type E. inaequidens. Middle Paleocene. 



Protoselene: Type P. opisthacus. Middle Paleocene. 



Litaletes: Type L. disjunctus. Middle Paleocene. 



Litomylus: Type L. dissentaneus. Middle Paleocene. 



Haplaletes: Type H. disceptatrix. Middle Paleocene. 



Litolestes: Type L. ignotus. Upper Paleocene. 



Phenacodaptes: Type P. sabulosus. Upper Paleocene. 



Haplomylus: Type //. speirianus. Uppermost Paleocene and Lower Eocene. 



Hyopsodus: Type H. paulus. Lower to Upper Eocene. 



The types and many other specimens of these genera have been 

 eexamined for the present work ^* in order fully to analyze the affini- 

 ties of the Fort Union forms here described and their contribution 

 to knowledge of the family. 



In the first instance, generic designations were disregarded and the 

 18 well-defined Paleocene species referable to this family were graphi- 

 cally compared by a tabulation of all their known characters. ^^ 

 They were found to fall naturally into groups that correspond very 

 well with the various genera recognized by Matthew and those 

 defined after his work. The principal characters distinguishing 

 these generic groups are shown in the following key, which also gives 

 (in parentheses) the species now placed in each genus, the type being 

 indicated by an asterisk. 



KEY TO THE PRINCIPAL GENERA OF HYOPSODONTIDAE KNOWN IN THE PALEOCENE 



I. Paraconids internal, fusing with metaconids; entoconids indis- 

 tinct, fusing with hypoconulids. 

 A. P^ enlarged, inflated, few accessory cuspules. 



1. Paraconids less internal, entoconid distinct on M2, few or no 



crenulations, M3 large, with projecting hypoconulid, M* 



with large metacone Choeroclaenus ^^ (turgidunculus*) 



2. Paraconids wholly internal, entoconid indistinguishable on 



M2, crests crenulated. M3 much reduced, with rounded 



heel. Metacone vestigial on M^ Mioclaenus (turgidus*) 



" Material of Litolestes and Phenacodaptes through the courtesy of Dr. Q. L. Jepsen. The American 

 Museum collections includemostof the types and many excellent specimensof all the genera except PAcnaco- 

 daptes. The type specimen of Hyopsodus paulus was not seen, but many good specimens of this and other 

 species of Hyopsodus were examined. 



'3 Two or three poorly known supposed species of doubtful status were omitted. 



'8 New genus, defined on p. 232. 



